This invention relates generally to intensity modulated radiation therapy for the treatment of cancer or the like, and specifically to a method for precisely delivering the dose of radiation to a displaced target.
Medical equipment for radiotherapy treats tumorous tissue with high energy radiation. The amount of radiation and its placement must be accurately controlled to ensure that the tumorous tissue receives sufficient radiation to be destroyed, and that the damage to the surrounding and adjacent non-tumorous tissue is minimized.
The radiotherapy process typically involves treatment planning and treatment delivery. The radiotherapy process commonly begins with the acquisition of three-dimensional patient images, such as a computed tomography (CT) image or a magnetic resonance image (MRI). Next, relevant anatomical structures are delineated or contoured. These structures can be classified as either target volumes to be irradiated, or sensitive structures to which radiation should be minimized. A treatment plan is then prepared that is optimized to maximize treatment to the target volumes while minimizing radiation to the sensitive structures.
Typically the tumor or target volume will be treated from several different angles with the intensity and shape of the radiation beam adjusted appropriately. The purpose of using multiple beams, which converge on the site of the tumor, is to reduce the dose to areas of surrounding non-tumorous tissue. The angles at which the tumor is irradiated are selected to avoid angles which would result in irradiation of particularly sensitive structures near the tumor site. The angles and intensities of the beams for a particular tumor form the treatment plan for the tumor.
Due to anatomical changes caused by tumor growth, breathing, organ movement or the like, the intended structures may not receive the planned dose, even if the radiation therapy machine operates perfectly. For example, the motion of target structures relative to sensitive structures could cause underdosing or overdosing of those structures, respectively. A “correct” patient set-up does not always result in the planned dose hitting the target. It would be beneficial to collect reliable patient images, or fraction images, immediately prior to treatment. Specifically, one could contour those images, and then optimize a new plan to treat the patient based on the current anatomical locations. However, this process is not generally fast enough to be performed on-line while the patient is on the treatment couch waiting to be treated. In principle, if reliable patient images or fraction images could be collected immediately prior to treatment, those images could be contoured, and then used to re-optimize a new plan to treat the patient based on current anatomical locations at the time of treatment. However, because the patient cannot remain motionless on the treatment couch for too long, this process is typically not fast enough, as any movement ultimately compromises newly acquired anatomical information.
One of the major efforts in radiotherapy has been in reducing the effect of treatment variations, such as beam placement errors and geometric variation of treatment targets and critical normal organs. To compensate for these variations, a predefined margin around the target volume is utilized. This is often referred to as the clinical target volume (CTV). Theoretically, the use of a margin will ensure that the target volume receives the intended dose, even if the target is somewhat displaced. Uncertainties in the treatment process often require the delineated tumor volume be enlarged by a margin to a planning target volume (PTV). This margin ensures that the tumor receives the intended target dose. Thus, through the long fractionated treatment, the tumor should always be contained within the PTV. However, too large a margin results in delivering unnecessary dose to healthy tissues, yet too small a margin could preclude the target from receiving the desired dose.
Over the course of radiotherapy treatment, the radiation is delivered in doses. A single dosage day is considered a dosage “fraction.” From day to day, or even more frequently, internal organ motion can cause the target volume to move. Such movement presents a very significant source of imprecision. The problem is exacerbated as increasingly conformal treatments are attempted, such as through the use of intensity modulated radiotherapy (IMRT). As margins become tighter around target volumes, the success of the treatment is increasingly dependent upon having the target volumes precisely situated in the intended locations. The use of high dose gradients to avoid radiation to sensitive structures means that slight shifts of these structures can subject them to large unintended doses.
The advent of on-line three-dimensional imaging for radiotherapy promises to vastly increase knowledge of patient anatomy at the time of treatment. It is important to have a knowledge of a patient's anatomy at the time of treatment. CT imaging is a particularly useful tool for verifying how a radiation dose was or is to be distributed with respect to the target volume and margin, and can detect if sensitive structures will be or are harmed by a shift in position. And since CT imaging is typically used for treatment planning, it is also an appropriate way to monitor the patient on a day-to-day basis. It would be particularly useful to have CT capability integrated into a radiotherapy system because this precludes the need to position a patient twice and thus, minimizes the likelihood of patient motion between imaging and treatment. It is also desirable to use image guidance for treatment verification before, during and after delivery to modify treatment plans. This process is referred to as image guided adaptive radiotherapy (IGART) and is an extension of adaptive radiotherapy (ART). In a broad sense, IGART seeks to incorporate feedback into the radiotherapy process to deliver the most appropriate treatment for a patient on any given day, and to remedy any imperfections in earlier fraction deliveries.
ART is a closed-loop radiation treatment process where the treatment plan can be modified using a systematic feedback of measurements. It improves radiation treatment by systematically monitoring treatment variations and incorporating them to reoptimize the treatment plan early on during the course of treatment. In the ART process, the field margin and treatment dose can be routinely customized to each individual patient to achieve a safe dose escalation.
Typical uses for on-line imaging includes rigid-body patient positioning, indication of changes in internal anatomy, calculation of delivered dose, either prospectively or retrospectively, and reoptimization of treatments, as necessary. Dose calculations and reoptimizations can be very time consuming, especially for complex IMRT treatment, making them virtually impossible to complete while the patient is on the treatment couch.
Accordingly, a need exists for a method for precisely delivering a dose of radiation to a tumor without the need for complete dose calculations or reoptimization.